Well, if it didn't need to be steelhead, there's still a couple of weeks left on the south fork of the Boise river below the dam before it closes. If you have a 6 wt or less you can swing light wets size 6 and smaller for 20+" rainbows, lots of great runs and no one around during the week. Flows are around 600cfs, which creates some nice runs. There are some primatve campsites along the river, plenty of sticks to eat.
I've caught steel on the Imnaha and Wallowa rivers in march, hatchery brats. The Imnaha is pretty remote, not many fisherman, smallish river but a great canyon river that comes off the snake river (both Oregon fisheries) and has some primatve camping iirc.